Calcium and magnesium phosphates tend to precipitate on surfaces of implant devices, a phenomenon called “encrustation.” See J. Urol. (1988) 139:37-38 and Br. J. Urol. (1994) 73:687-691. It has been suggested that encrustation on surfaces of urethral stents is mainly due to (1) the high pH of urine, which promotes formation of calcium and magnesium phosphates; (2) growth of bacteria, which convert urea in urine to ammonia, thereby further increasing the pH of urine; and (3) rough and hydrophilic surfaces, which create nuclei sites for encrustation. Such encrustation frequently leads to blockage and fracture of implant devices.